Combinatorial processing has been used to evaluate materials, processes, and devices formed in semiconductor processing as well as other industries such as batteries, catalysts, pharmaceuticals, and biotechnology. Significant efforts to apply combinatorial processing to solar applications have not been made.
Solar cells have been widely researched as clean renewable energy sources are needed. Currently, mono and multi-crystalline silicon solar cells are the dominant technologies in the market. Mono and multi-crystalline silicon solar cells require a thick layer of silicon to efficiently absorb sun light. This determines the low material utilization rate for crystalline silicon solar cells, hence the difficulties of achieving low cost electricity. Thin film solar cells, on the other hand, require a very thin layer of absorber material due to their inherent material properties to efficiently convert sunlight directly to electricity. Amorphous/microcrystalline silicon (Si), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) are three leading materials used in thin film solar cells that are currently in production. Among these three thin film solar cells, CIGS has the best lab cell efficiency (close to 20%) and the best large area module efficiency (>12%).
In particular, development of solar cell test substrates using vacuum processing tools for the combinatorial evaluation of copper-zinc-tin-sulfide (Cu2ZnSnS4) (CZTS) solar cells and CIGS solar cells has not been done. Improvements, whether in materials, unit processes, or process sequences, are continually being sought for the solar cells. However, solar companies conduct research and development (R&D) on full substrate processing. This approach has resulted in escalating R&D costs and the inability to conduct extensive experimentation in a timely and cost effective manner.
The increasing demand for environmentally friendly, sustainable and renewable energy sources is driving the development of large area, thin film (TF) photovoltaic (PV) devices. With a long-term goal of providing a significant percentage of global energy demand, there is a concomitant need for Earth-abundant, high conversion efficiency materials for use in photovoltaic devices. A number of Earth abundant direct-bandgap semiconductor materials now seem to show evidence of the potential for both high efficiency and low cost in Very Large Scale (VLS) production (e.g. greater than 100 gigawatt (GW)), yet relatively little attention has been devoted to their development and characterization.
Among all thin film PV technologies, CIGS and CdTe are the only two materials that have reached volume production with greater than 10% stabilized module efficiencies. Solar cell production volume must increase tremendously in the coming decades to meet sharply growing energy needs that are expected to double to 27 terawatts (TW) in 2050. However, the supply of indium (In), gallium (GA) and tellurium (Te) may inhibit annual production of CIGS and CdTe solar panels. Moreover, price increases and supply constraints in indium and gallium could result from the aggregate demand for these materials used in flat panel displays (FPD) and light-emitting diodes (LED) along with CIGS PV. Efforts to develop devices that leverage manufacturing and R&D infrastructure related to thin film PV using more widely available raw materials should be considered a top priority for the world.
CZTS kesterites are one type of Earth-abundant material system that is garnering increasing interest from the PV community. IBM recently announced a 9.66% conversion efficiency Cu2ZnSn(Se,S)4 (CTZSS) solar cell. This solar cell used spin coated films from hydrazine-based solutions; however, hydrazine is a known toxin. Also, the cell used cadmium in the CdS buffer layer and a magnesium fluoride anti-reflective coating. While this set of materials may not be desirable for manufacturing, the sharp efficiency improvement over a very short period of time illustrates the potential of CZTS-type materials for PV. Still, there exists a major gap between this proven performance and the theoretical single junction efficiency of 32%.
The immaturity of thin film PV devices exploiting Earth abundant materials represents a daunting challenge in terms of the time-to-commercialization. That same immaturity also suggests an enticing opportunity for breakthrough discoveries. A quaternary system such as CZTS requires management of multiple kinetic pathways, thermodynamic phase equilibrium considerations, defect chemistries, and interfacial control. The vast phase-space to be managed includes process parameters, source material choices, compositions, and overall integration schemes. Traditional R&D methods are ill-equipped to address such complexity, and the traditionally slow pace of R&D could limit any new material from reaching industrial relevance when having to compete with the incrementally improving performance of already established thin film PV fabrication lines.
However, due to the complexity of the material, cell structure and manufacturing process, both the fundamental scientific understanding and large scale manufacturability are yet to be improved for CZTS and CIGS solar cells. As the photovoltaic industry pushes to achieve grid parity, much faster and broader investigation is needed to explore the material, device and process window for higher efficiency and a lower cost of manufacturing process. Efficient methods for forming different types of CZTS solar cells and CIGS solar cells that can be combinatorially varied and evaluated are necessary.